Squeezed Thermal States Research Articles

Decoherence and Fidelity in Teleportation of Coherent Photon-Added Two-Mode Squeezed Thermal States

We theoretically introduce a kind of non-Gaussian entangled resources, i.e., coherent photon-added two-mode squeezed thermal states (CPA-TMSTS), by successively performing coherent photon addition operation to the two-mode squeezed thermal states. The normalization factor related to bivariate Hermite polynomials is obtained. Based upon it, the nonclassicality and decoherence process are analyzed by virtue of the Wigner function. It is shown that the coherent photon addition operation is an effective way in generating partial negative values of Wigner function, which clearly manifests the nonclassicality and non-Gaussianity of the target states. Additionally, the fidelity in teleporting coherent states using CPA-TMSTS as entangled resource is quantified both analytically and numerically. It is found that the CPA-TMSTS is an entangled resource of high-efficiency and high-fidelity in quantum teleportation.

Analysis of electromagnetically induced transparency based on quantum memory of squeezed state of light

Quantum memory of light is not only the building block of constructing large-scale quantum computer, but also the kernel component of quantum repeater for quantum networks, which makes long distance quantum communication come true. Due to the inevitable optical losses, squeezed vacuum generated from optical parametric amplifier becomes squeezed thermal state of light, which is no longer the minimum uncertainty state. Therefore quantum memory of squeezed thermal state of optical field is the key step towards the implementation of quantum internet. Atomic ensemble is one of ideal quantum memory media, as a result of high optical depth and good atomic coherence. Electromagnetically induced transparency (EIT) is one of mature approaches to quantum state mapping between non-classical optical fields and atomic spin waves. In atomic ensembles, the EIT can on-demand map quantum state between quadratures of light and spin waves of atomic ensemble, i.e., controlled quantum memory. Here the condition of quantum memory for squeezed thermal state of light is investigated according to the fidelity benchmark of quantum memory. The fidelity benchmark of quantum memory is the maximum fidelity which can be reached by classical methods, and it is quantum memory if the memory fidelity is higher than the fidelity benchmark of quantum memory. By numerically calculating the fidelity benchmark of quantum memory for different kinds of squeezed thermal states of light and the dependence of memory fidelity on the memory efficiency, we obtain the minimum memory efficiency which can realize quantum memory for squeezed thermal state of light. The quantum memory can be easily obtained by increasing squeezing parameter r. The thermal state fluctuation is sensitive to the realization of quantum memory. The required minimum memory efficiency is lower, when smaller thermal state fluctuation is employed in experiment by reducing the optical losses in optical parametric amplifier. On the other hand, quantum memory fidelity benchmark is high for small squeezing parameter and large optical depth, which requires high memory efficiency. And atomic memory efficiency can be increased by utilizing optical cavity to enhance the interaction between light and atom or atomic ensemble with high optical depth. For example, the fidelity benchmark is 0.80, when squeezing parameter r is 0.35 and thermal state fluctuation is 2.38 dB. Thus quantum memory can be realized if the memory efficiency is larger than 4.34%. Our work can provide the direct reference for experimental design of continuous variable quantum memory, quantum repeater, and quantum computer based on atomic ensembles.

Non-Markovian Dynamic of Gaussian Quantum Discord in Continuous-Variable Systems

We show the special dynamic characteristic of Gaussian quantum discord, exhibited by two-mode Gaussian symmetric squeezed thermal states (STS) in continuous-variable (CV) systems with a common non-Markovian environment, is de nitely di erent from Markov processes. We demonstrate that Gaussian quantum discord can be created whenever the information ow from environment back to the system. We also show that the rate of decrease for Gaussian quantum discord is related to the coupling constant. We discover that the initial value of Gaussian quantum discord is determined by the average number of thermal photons of the system.

Nonlocal Correlations of Moving Three-level Atoms in Photon-added Squeezed Thermal States

We investigate nonlocal correlations between a three-level atom and the optical field initially being in the photon-added squeezed thermal state. We examine the effect of the squeezing parameter and the number of added photons to the squeezed thermal field on the entanglement during the time evolution. We show that the entanglement can be enhanced by increasing the number of added photons and the squeezing parameter for moving and nonmoving three-level atoms. We present a useful way to generate and restrain a large amount of entanglement by increasing the number of added photons when the atomic motion is described by the atomic velocity.

Nonclassicality and Decoherence of Generalized Photon-Modulated Squeezed Thermal State

In this paper, we introduce the generalized photon-modulated squeezed thermal states (GPMSTS) obtained by repeatedly acting the combination of Bosonic creation and annihilation operation on the squeezed thermal state. We investigate the nonclassical properties of GPMSTS and its decoherence in a thermal environment according to the negative region of Wigner function. It is found that the normalization factor of GPMSTS is a Legendre polynomial. Especially, the decoherence effect is discussed by deriving the time evolution of the Wigner function.

Quantum discord for Gaussian states with non-Gaussian measurements

In recent years the paradigm based on entanglement as the unique measure of quantum correlations has been challenged by the rise of new correlation concepts, such as quantum discord, able to reveal quantum correlations that are present in separable states. It is in general difficult to compute quantum discord, because it involves a minimization over all possible local measurements in a bipartition. In the realm of continuous variable (CV) systems, a Gaussian version of quantum discord has been put forward upon restricting to Gaussian measurements. It is natural to ask whether non-Gaussian measurements can lead to a stronger minimization than Gaussian ones. Here we focus on two relevant classes of two-mode Gaussian states: squeezed thermal states (STS) and mixed thermal states (MTS), and allow for a range of experimentally feasible non-Gaussian measurements, comparing the results with the case of Gaussian measurements. We provide evidence that Gaussian measurements are optimal for Gaussian states.

New approach for normalization and photon-number distributions of photon-added (-subtracted) squeezed thermal states

Using the thermal field dynamics theory to convert the thermal state to a "pure" state in doubled Fock space, it is found that the average value of e^{fa^{{\dag}}a} under squeezed thermal state (STS) is just the generating function of Legendre polynomials, a remarkable result. Based on this point, the normalization and photon-number distributions of m-photon added (or subtracted) STS are conviently obtained as the Legendre polynomials. This new concise method can be expanded to the entangled case.

Coherent operation of photon subtraction and addition for squeezed thermal states: analysis of nonclassicality and decoherence

We study nonclassical properties of the optical field when photon subtraction/addition coherent operation ta+ra† (|t|2+|r|2=1) acts on a squeezed thermal state (STS) by examining its quadrature squeezing, sub-Poissonian statistics, and negativity of the Wigner function. The degree of squeezing of the coherent operated squeezed thermal state (COSTS) becomes weaker. The Mandel Q parameter decreases with r and quickly becomes negative, which implies sub-Poissonian statistics. Both the negative dip and negative area of the Wigner function increase with r for small squeezing values of κ. Decoherence of the COSTS in an amplitude-damping channel is studied by the time evolution of the Wigner function. The length of time that this nonclassical field preserves its partial negativity of the Wigner function can be modulated by the coherent operation. All these results indicate that the nonclassicality of the STS is sensitive to the coherent operation.

Quantum Entanglement and Nonlocality Properties of Two-Mode Squeezed Thermal States in a Common-Reservoir Model

We study a system consisting of two identical non-interacting single-mode cavity fields coupled to a common vacuum environment and provide general, explicit, and exact solutions to its master equation by means of the characteristic function method. We analyze the entanglement dynamics of two-mode squeezed thermal state in this model and show that its entanglement dynamics is strongly determined by the two-mode squeezing parameter and the purity. In particular, we find that two-mode squeezed thermal state with the squeezing parameter is extremely fragile and almost does not survive in a common vacuum environment. We investigate the time evolution of nonlocality for two-mode squeezed thermal state in such an environment. It is found that the evolved state loses its nonlocality in the beginning of the evolution, but after a time, the revival of nonlocality can occur.